This invention relates a manufacturing method of an electrode for a discharge lamp (hereinafter referred to as "discharge lamp electrode", when applicable).
In general, a discharge lamp employs tungsten discharge electrodes. Such a tungsten electrode is usually in the form of a rod having a predetermined length. Formerly, the end face of a rod-shaped electrode, which is a discharge part and is held opposite the end face of the other electrode in the discharge space, was an as-cut face of an electrode forming material. In this case, burrs are likely to remain on the cut face, i.e., on the discharge part of the electrode, thus giving rise to the following problem. The initial arc spot is concentrated on the burr, and may move along the burr due to, for instance, vibrations of a vehicle in which the discharge lamp is installed, thus making the discharge operation unstable. In order to overcome this problem, various methods have been proposed in the art to work the end portion of the electrode into a spherical shape.
In one of such methods (hereinafter referred to as "first prior art method", when applicable) as shown in FIG. 5, the end portion of a rod-shaped electrode material 2 is worked to have a spherical surface using a laser beam 3.
In another method (hereinafter referred to as "second prior art method", when applicable) as shown in FIG. 6, discharges are induced between two rod-shaped electrode materials 6 and 6' the end portions of which confront each other, to form the end portions into spherical end portions 6aand 6a'.
However, the first prior art method involves the following problems, because in order to make spherical the end portion of the rod-shaped material 2 it is essential to apply the laser beam 3 to the material 2 while rotating the material 2. This necessitates a rotating mechanism 4 and adjustments of the rotational speed of the rod-shaped material 2 and the intensity of the laser beam 3. That is, the apparatus for practicing this method becomes intricate in construction, and is not suitable for mass-production of discharge lamp electrodes.
In the second prior art method, it is necessary to provide a container 8 for forming a discharge space, and it is considerably troublesome to attach or detach the rod-shaped materials 6 and 6' placed in the container 8. In particular, electrodes for a small metal halide lamp have small dimensions in diameter and in length (about 5 mm in length). Thus, the second prior art method is not suitable for mass-production of discharge lamp electrodes either.